1. Field of the Invention
The invention relates generally to display systems, and more particularly to a display having a plurality of characters, each of which is composed of plurality of segments which are electrically unidirectional, e.g. Light Emitting Diode.
2. Description of the Prior Art
Prior Art displays are well known. A typical example is the numerical display in a hand-held calculator. There are typically 8 characters or digits. Each character is made of 7 segments which are arranged in the shape of an "8". If all the segments are illuminated, then the number "8" is displayed, and by selectively illuminating different combinations, the digits from 0-9 are displayed. Segments may be either unidirectional, such as LED's, or bi-directional, such as liquid crystals. The present invention relates to the unidirectional type segment.
In the prior art, the segments of each character are turned on, or illuminated, in a multiplexed fashion. For example, the human eye has a persistence of about 50 milliseconds. The segments representing the number to be displayed on the first character of the display are turned on for 2 or 3 milliseconds. Then the segments corresponding to the number to be displayed in the second character are turned on or illuminated in the next 2 or 3 milliseconds; and so on for each character in the display. After the last character in the display, the drive then repeats by illuminating the first character. In this arrangement the segments in each of the characters are turned on for a few milliseconds. The human eye interprets this as a continuously ON light, so long as the segments in each display are refreshed or reilluminated more often than every 50 milliseconds, which is the presistence of the human eye.
In the prior art display systems there are a large number of leads connecting the multiplex drive circuit and the displays. For example, in a display having 8 characters, each character having 7 segments; there are 7 leads connecting corresponding segments of the characters in parallel; and 8 strobe leads, one to each character in the display. Signals as to which segments are to be illuminated in a character go out on the 7 segment leads, and a signal as to which character in the display is to display that number is carried on the 8 strobe leads. Coincidence on the two signals at a particular character causes those segments in that character to illuminate. The number of leads can be generalized as equal to the number of segments in each character; plus the number of characters in the display. In the example above, this number is 15.
It is a desirable and classical goal to reduce the number of leads between the drive circuit and the display. There are several reasons to reduce the number of leads to a minimum. First, as the drive circuits are made on an integrated circuit chip and these chips get smaller and more functions are put on a single chip, the amount of space required for pads needed to connect to the leads should also be reduced. Fewer leads and pads means fewer interconnections between pads on the integrated circuit chip and the I.C. housing, and fewer pins on the I.C. housing. Next the number of interconnected lead patterns on the drive circuit are also reduced. This reduces labor and material cost, but also improves quality, as there are fewer errors and a higher yield with fewer connections. Finally, fewer interconnections means less radio frequency interference.
The present invention is an interconnection of leads between unidirectional segmented display and a multiplexed drive circuit in which the number of leads are reduced from what was heretofore required. For example in the 7-segment 8-character display, only 8 leads are required, a reduction of almost 50%. A 16-segment (alphanumerical symbol) 16-character display requires only 17 leads, while the prior art requires 32.
The present invention achieves the goal of a reduced number of interconnections between display and drive circuit while using the same multiplexed rates and duty cycles as conventional multiplexed display systems. It achieves the goal of reduced number of leads, reduced number of pin-out in the display drive circuit (typically in a DIP package), and a corresponding reduced number of pads on the integrated circuit chip. The likelihood or chance of radio frequency interference is reduced due to less interconnections. There is a potential of lower cost of LED display modules and the associated drive circuit, as well as improved quality and yield with a higher number of good units manufactured.
An object of the invention is to provide an improved multiplexed display system having a unilateral segmented characters in which the number of leads between the drive circuit and the display is reduced.
Another object of the invention is to provide a novel display which is easier to assemble and repair, and which has a higher yield due to a reduced number of interconnections between the drive circuit and the display.